1. Field of the Invention
This invention relates to the treatment of waste effluent and more particularly to method of treating waste effluent from the manufacture of collagen food casings to remove and recycle aluminum salts present therein.
2. The Prior Art
Collagen casings have been widely accepted as edible food casings for pork sausages. In the preparation of edible collagen casings, hide collagen derived from bovine hides is converted into a finely divided fibrillar form and extruded in the form of a dilute collagen slurry. Usually, the extruded collagen is passed into a sodium sulfate or ammonium sulfate coagulating bath which dehydrates the collagen slurry and forms a coherent tubular collagen film. At this stage in the processing, the salt coagulated collagen film can be handled. However, removal of the ammonium sulfate or other coagulating salt from the film by washing will cause the collagen film to revert to a paste or slurry. It is therefore necessary to harden or tan the extruded collagen film to permit further processing of the film through the steps of washing, drying, shirring and stuffing.
One of the most widely used methods of tanning collagen casings is described in U.S. Pat. No. 3,123,481 which utilizes a tanning bath containing 3 to 18% by weight of a citrato complex of aluminum ammonium sulfate (alum). One drawback to this tanning process is that a small portion of the alum present in the bath eventually finds its way into process effluent and must be removed therefrom as its presence evokes a pollution problem. It is also desirable to recover the aluminum salt because of its economic value.
Most processes for the removal of soluble aluminum ion from effluent streams are generally based on the precipitation of the aluminum ion as the insoluble hydroxide. When the aluminum ion is present in small amounts, e.g., in the order 200-400 ppm, no problems are encountered with the precipitation method. Unfortunately when the aluminum concentration in the effluent stream increases to more substantial concentrations, e.g., in the order of 0.25-1.0% or more, as is the case in effluent from collagen tanning baths, the hydroxide precipitates in the form of a hydrated gel which is difficult to filter and separate from the effluent, the gel that forms often comprises half the volume of the effluent undergoing aluminum ion precipitation. In process effluent from the manufacture of collagen casings, the effluent also generally contains a high concentration, i.e. in the order of about 3% of other salts e.g. ammonium sulfate, ammonium lactate and ammonium citrate and these salts become trapped in the gel and further compound the problem of separating the gel from the effluent steam. Due to the contamination of the gel with these other salts, recycling of the aluminum salt is deterred.